Heat exchangers such as steam generators and, in particular pressurized-water nuclear reactor steam generators, generally comprise a bundle of very long small-diameter tubes constituting the exchange surface, allowing the feed water of the steam generator to be heated and vaporized.
In a nuclear power station, the reactor of which is cooled and moderated by pressurized water, the heat released by the nuclear reaction is extracted from the core by the primary coolant and transferred in the steam generator to the feedwater which, after vaporization, drives the turboalternator sets of the power station. This feedwater is sent in liquid form into the steam generator, after passing through the condenser.
The exchange surface of a steam generator of a pressurized-water nuclear reactor consists of a large number of tubes (for example, 3400 tubes for each of the three steam generators of a 900 MW.e power station), inside which tubes the primary coolant flows. The feedwater comes into contact with the external surface of the tubes.
The tubes have an internal diameter of approximately 20 mm and are fixed at each of their ends in bores passing through a tube plate having a thickness of the order of 550 mm.
The connection between the tube and the tube plate is provided by expansion of the tube in a corresponding penetrating bore in the plate and by a weld at its lower end.
In addition, the tubes are held transversely by spacer plates which are drilled with holes for passage of the tubes and are distributed along the length of the steam-generator tubes so that any two spacer plates are separated by a distance of approximately one meter.
The tubes of the bundle of a steam generator constitute not only the surface for heat exchange between the primary coolant and the feedwater but also a wall for confining the primary coolant performing an extremely important role as regards the safety of operation of the nuclear plant.
In the case of a power station comprising a pressurized-water reactor having a power of 900 MW.e, the primary coolant is at a pressure close to 155 bar and at a temperature of 300.degree. C., and the feedwater is at a pressure of 56 bar and at a temperature of 271.degree. C.
As a result of the pressure difference existing between the primary coolant and the feedwater, damage of a tube of the generator bundle may give rise to leakage of primary coolant into the feedwater. The primary coolant is laden with substances in solution or in suspension which are radioactive and consequently even a very slight leak in a tube of the steam-generator bundle leads to contamination of the feedwater and of the power station components in which this feedwater circulates. Such a defective operating regime is not acceptable insofar as the feedwater circulates outside the safety buildings of the nuclear reactor in the turbine set and in all the equipment and auxiliary circuits which are associated with this set.
The tubes of the bundle of a steam generator are designed and sized so that they can withstand without damage the various mechanical and thermal loads to which they are subjected in service. The material of which they are made is defined so as to prevent, as far as possible, corrosion of these tubes by the fluids with which the tubes come into contact.
Furthermore, the chemical properties of the primary coolant and of the feedwater are, during the operation of the plant, checked continuously and possibly rectified, so as to reduce the risks of corrosion.
However, it is necessary to ensure at all times that the tube bundle of the steam generator is in a satisfactory state and provides perfect separation of the primary coolant from the feedwater. This checking is carried out by continuous monitoring, in operation, of the level of activity of the feedwater, which makes it possible to detect leaks whose flow rate is very low. During the periods when the nuclear plant is shut down, an examination of the tubes of the bundle is carried out, usually by eddy currents, so as to detect defects whose development could subsequently lead to a leak. This eddy-current examination, which is necessary to ensure satisfactory operation of the steam generator, must, after the nuclear plant has been started up again, be able to be carried out under very good conditions in order to detect any defect which could lead to the appearance of a leak in the steam generator.
Despite the various precautions taken, both during design and manufacture and during the operation of the steam generators, it has turned out that certain materials used for the manufacture of the tubes of the bundle had quite a high sensitivity to stress corrosion, on the primary side. This is the case, in particular, for certain grades of nickel-based alloys containing chromium and iron.
Other types of degradation have also been observed on the feedwater side, such as wear by loose parts or by intergranular corrosion (IGA) or stress corrosion both in the region of the tube plate and of the spacer plates and, exceptionally, in the spanning parts of the tubes.
Stress corrosion develops mainly in the regions where the tube exhibits residual stresses and, in these regions, a crack may form through the thickness of the tube, which is liable finally to result in a leak of the primary coolant into the feedwater.
One region particularly sensitive to this type of corrosion is at the upper face of the tube plate. The reason for this is that the tube, after being inserted into the tube plate and before its lower end is welded, is subjected to a diametral expansion operation called mandrelling or tube expanding and whose purpose is to ensure intimate contact between the external surface of the tube and the surface of the bore drilled in the tube plate.
When the tube has been crimped by mandrelling over the entire thickness of the tube plate, there remains in the wall of the tube a transition zone between the mandrelled part of the tube in contact with the bore of the tube plate and the upper part of the tube which has not been subjected to the diametral expansion. In this transition zone, there are residual stresses in the tube which, in the case where the material is sensitive to stress corrosion, may give rise to intergranular cracking, the growth of which could consequently lead to a leak of primary coolant through the thickness of the tube.
In order to remedy this drawback, methods have been proposed for thermally or mechanically stress-relieving the wall of the tubes of the bundle of a steam generator in the transition zone.
However, it is necessary also to use repair methods which can be employed on steam generators whose tube bundle has already been subjected to degradation.
Certain sleeving methods consist in fixing a sleeve to part of the internal surface of the tube in such a way that the sleeve (or sleeve lining) conceals the crack that has penetrated the wall of the tube or risks penetrating this wall.
The sleeve, whose diameter is less than the internal diameter of the tube, is placed in the desired position inside this tube and subjected to diametral expansion by mandrelling which ensures both the mechanical integrity and the sealing of the fixing of the sleeve. Mandrelling may be carried out over the entire height of the sleeve or just at the upper and lower ends of the sleeve.
The sleeve may also be brazed inside the tube or fixed by a weld bead at each of its ends.
In certain cases, one end, preferably the lower end, of the sleeve is fixed by mandrelling in the tube, and the other end of the sleeve is fixed by welding.
Prior art sleeving methods make it possible effectively to repair tubes having defects resulting from cracks that have grown by stress corrosion and to prevent leaks of primary coolant into the feedwater. However, after a certain operating time with the tubes repaired in this way, it has been observed that the tube bundle had once again a certain leak rate detected by monitoring the radioactivity of the feedwater. Upon examination, it seemed that new defects had developed in the tubes, generally at or near the upper end where the sleeve was fixed in the tube.
The upper end of the lining sleeves, which lies in that part of the tube projecting from the upper face of the tube plate and which is generally fixed by crimping inside the tube, is located precisely in a region where the tube undergoes a certain diametral expansion and where it exhibits a high stress concentration.
The sleeve and the tube exhibit variations in diameter (expansions), in shape and in the nature of the contacts (mechanical, welding or brazing), which make it difficult to carry out the conventional checks using eddy-current probes (integrating probes or rotating probes).
Overall, it is observed that sleeving has two serious drawbacks:
1--in order to ensure bonding between the tube and the sleeve, the corrosion-sensitive tube is deformed and the stresses induced by this deformation lead to a risk of future corrosion in the newly deformed region; PA1 2--the assembly thus formed is very difficult to check because of the complex geometry of the repair. PA1 3--a thin layer of nickel protects just the primary side of the tube, but it is too thin to take up the mechanical forces if the tube no longer provides this function because of damage initiated on the external side of the tube; PA1 4--nickel is ferromagnetic, which prevents the tube from being checked using conventional eddy-current methods and requires the addition of a new operation of checking using ultrasound at each shutdown, which results in a high penalty from a cost and a delay standpoint. PA1 the plated metal layer has a thickness sufficient to withstand by itself the forces exerted on the tube in service; PA1 the metal layer has a thickness of between 0.5 and 1.5 mm; PA1 the metal layer has a length of between 100 and 200 mm in the axial direction of the tube; PA1 the plated metal layer is formed by an alloy of nickel and boron, the boron content of which is less than 5% by weight; PA1 the boron content of the nickel alloy is less than 0.5% by weight; PA1 the electroplating is carried out with a pulsed current; PA1 the pulsed current has a frequency of between 50 and 1000 hertz and preferably approximately 100 hertz. PA1 a) in order to overcome the obstacle represented by the ferromagnetic character of nickel with respect to the eddy-current checks, a novel plating material has been chosen which may be formed, for example, of nickel weakly enriched with boron, knowing that Ni-B platings are virtually non-magnetic and therefore allow checking using eddy currents; PA1 b) in order to deposit the equivalent of a sleeve, but without deformation of the tube, which is prejudicial both to corrosion in the deformed region and to the checking of the complex geometry thus formed, only an electroplating method is used in order to deposit a sleeve (for example made of Ni-B) with a large thickness which allows it to take up the mechanical forces and the pressure, even if damage to the tube were to continue via the outside and even if this were to lead to complete cut-off of the tube in the damaged region.
A method is known, described in FR-A-2,565,323, which makes it possible to protect from stress corrosion a tube such as a steam-generator tube crimped in a tube plate, in particular this tube's transition zone located in the vicinity of the outlet face of the tube plate and corresponding to the region which separates the part of the tube which is expanded inside the tube plate from the part of the tube which is not expanded. This protection method consists in electroplating the internal surface of the tube with a thin metal layer after the tube has been fixed in the tube plate. The electroplating makes it possible to isolate the internal surface of the tube, in particular in the region where the wall of the tube has a high stress concentration, from the exchange fluid such as the pressurized water circulating inside the tube.
Such a method, using nickel plating, has already been widely used. It has two serious drawbacks: